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 * Copyright (c) 2012, 2013, Oracle and/or its affiliates. All rights reserved.
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
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 */
package java.util;

import java.util.function.DoubleConsumer;
import java.util.stream.Collector;

/**
 * A state object for collecting statistics such as count, min, max, sum, and
 * average.
 *
 * <p>This class is designed to work with (though does not require)
 * {@linkplain java.util.stream streams}. For example, you can compute
 * summary statistics on a stream of doubles with:
 * <pre> {@code
 * DoubleSummaryStatistics stats = doubleStream.collect(DoubleSummaryStatistics::new,
 *                                                      DoubleSummaryStatistics::accept,
 *                                                      DoubleSummaryStatistics::combine);
 * }</pre>
 *
 * <p>{@code DoubleSummaryStatistics} can be used as a
 * {@linkplain java.util.stream.Stream#collect(Collector) reduction}
 * target for a {@linkplain java.util.stream.Stream stream}. For example:
 *
 * <pre> {@code
 * DoubleSummaryStatistics stats = people.stream()
 *     .collect(Collectors.summarizingDouble(Person::getWeight));
 * }</pre>
 *
 * This computes, in a single pass, the count of people, as well as the minimum,
 * maximum, sum, and average of their weights.
 *
 * @implNote This implementation is not thread safe. However, it is safe to use {@link
 * java.util.stream.Collectors#summarizingDouble(java.util.function.ToDoubleFunction)
 * Collectors.toDoubleStatistics()} on a parallel stream, because the parallel implementation of
 * {@link java.util.stream.Stream#collect Stream.collect()} provides the necessary partitioning,
 * isolation, and merging of results for safe and efficient parallel execution.
 * @since 1.8
 */
public class DoubleSummaryStatistics implements DoubleConsumer {

  private long count;
  private double sum;
  private double sumCompensation; // Low order bits of sum
  private double simpleSum; // Used to compute right sum for non-finite inputs
  private double min = Double.POSITIVE_INFINITY;
  private double max = Double.NEGATIVE_INFINITY;

  /**
   * Construct an empty instance with zero count, zero sum,
   * {@code Double.POSITIVE_INFINITY} min, {@code Double.NEGATIVE_INFINITY}
   * max and zero average.
   */
  public DoubleSummaryStatistics() {
  }

  /**
   * Records another value into the summary information.
   *
   * @param value the input value
   */
  @Override
  public void accept(double value) {
    ++count;
    simpleSum += value;
    sumWithCompensation(value);
    min = Math.min(min, value);
    max = Math.max(max, value);
  }

  /**
   * Combines the state of another {@code DoubleSummaryStatistics} into this
   * one.
   *
   * @param other another {@code DoubleSummaryStatistics}
   * @throws NullPointerException if {@code other} is null
   */
  public void combine(DoubleSummaryStatistics other) {
    count += other.count;
    simpleSum += other.simpleSum;
    sumWithCompensation(other.sum);
    sumWithCompensation(other.sumCompensation);
    min = Math.min(min, other.min);
    max = Math.max(max, other.max);
  }

  /**
   * Incorporate a new double value using Kahan summation /
   * compensated summation.
   */
  private void sumWithCompensation(double value) {
    double tmp = value - sumCompensation;
    double velvel = sum + tmp; // Little wolf of rounding error
    sumCompensation = (velvel - sum) - tmp;
    sum = velvel;
  }

  /**
   * Return the count of values recorded.
   *
   * @return the count of values
   */
  public final long getCount() {
    return count;
  }

  /**
   * Returns the sum of values recorded, or zero if no values have been
   * recorded.
   *
   * If any recorded value is a NaN or the sum is at any point a NaN
   * then the sum will be NaN.
   *
   * <p> The value of a floating-point sum is a function both of the
   * input values as well as the order of addition operations. The
   * order of addition operations of this method is intentionally
   * not defined to allow for implementation flexibility to improve
   * the speed and accuracy of the computed result.
   *
   * In particular, this method may be implemented using compensated
   * summation or other technique to reduce the error bound in the
   * numerical sum compared to a simple summation of {@code double}
   * values.
   *
   * @return the sum of values, or zero if none
   * @apiNote Values sorted by increasing absolute magnitude tend to yield more accurate results.
   */
  public final double getSum() {
    // Better error bounds to add both terms as the final sum
    double tmp = sum + sumCompensation;
    if (Double.isNaN(tmp) && Double.isInfinite(simpleSum))
    // If the compensated sum is spuriously NaN from
    // accumulating one or more same-signed infinite values,
    // return the correctly-signed infinity stored in
    // simpleSum.
    {
      return simpleSum;
    } else {
      return tmp;
    }
  }

  /**
   * Returns the minimum recorded value, {@code Double.NaN} if any recorded
   * value was NaN or {@code Double.POSITIVE_INFINITY} if no values were
   * recorded. Unlike the numerical comparison operators, this method
   * considers negative zero to be strictly smaller than positive zero.
   *
   * @return the minimum recorded value, {@code Double.NaN} if any recorded value was NaN or {@code
   * Double.POSITIVE_INFINITY} if no values were recorded
   */
  public final double getMin() {
    return min;
  }

  /**
   * Returns the maximum recorded value, {@code Double.NaN} if any recorded
   * value was NaN or {@code Double.NEGATIVE_INFINITY} if no values were
   * recorded. Unlike the numerical comparison operators, this method
   * considers negative zero to be strictly smaller than positive zero.
   *
   * @return the maximum recorded value, {@code Double.NaN} if any recorded value was NaN or {@code
   * Double.NEGATIVE_INFINITY} if no values were recorded
   */
  public final double getMax() {
    return max;
  }

  /**
   * Returns the arithmetic mean of values recorded, or zero if no
   * values have been recorded.
   *
   * If any recorded value is a NaN or the sum is at any point a NaN
   * then the average will be code NaN.
   *
   * <p>The average returned can vary depending upon the order in
   * which values are recorded.
   *
   * This method may be implemented using compensated summation or
   * other technique to reduce the error bound in the {@link #getSum
   * numerical sum} used to compute the average.
   *
   * @return the arithmetic mean of values, or zero if none
   * @apiNote Values sorted by increasing absolute magnitude tend to yield more accurate results.
   */
  public final double getAverage() {
    return getCount() > 0 ? getSum() / getCount() : 0.0d;
  }

  /**
   * {@inheritDoc}
   *
   * Returns a non-empty string representation of this object suitable for
   * debugging. The exact presentation format is unspecified and may vary
   * between implementations and versions.
   */
  @Override
  public String toString() {
    return String.format(
        "%s{count=%d, sum=%f, min=%f, average=%f, max=%f}",
        this.getClass().getSimpleName(),
        getCount(),
        getSum(),
        getMin(),
        getAverage(),
        getMax());
  }
}
